RESPIRATION. 113 



sels and bronchial tube, connected to each other by cellular tissue, 

 and invested by the pleura. The bronchial tube is situated posterior 

 and superior to the pulmonary vessels ; the two pulmonary veins 

 are placed anterior and inferior to the artery and bronchus, and the 

 pulmonary artery is placed between the bronchus and the pulmonary 

 veins, but behind the pulmonary veins and before the bronchus. 

 The root of each lung has anterior to it the phrenic nerve and fila- 

 ments of the pneumo-gastric nerve ; posterior to it the pulmonic 

 plexus, and inferior the ligamentum latum. The root of the right 

 lung has the venaazygos arching over it. 



The right and left lungs differ from each other in some important 

 particulars : the right lung is broader and shorter than the left, and 

 consists of three lobes, separated by two fissures ; the right also 

 ascends higher in the neck, and the anterior edge of the left pre- 

 sents a notch where it corresponds to the apex of the heart. 



The intimate structure of the lungs consists of the ultimate rami- 

 fications of the bronchial tubes, which are the continuations of the 

 trachea, and the branches of the pulmonary artery and veins ; they 

 also receive bronchial arteries for their nutrition. 



TRACHEA AND ITS RAMIFICATIONS. 



The wind-pipe, or trachea, is a cylindrical tube, extending from 

 the crycoid cartilage of the larynx to the level of the third dorsal 

 vertebra. It consists of from seventeen to twenty fibro-cartilagi- 

 nous rings, truncated behind, and connected to each other by an 

 elastic membrane ; about the posterior fourth of each ring is de- 

 ficient, and its place is supplied by fibrous membrane. 



Opposite the third dorsal vertebra the trachea divides into the 

 right and left bronchial tubes ; the right bronchus is larger than the 

 left, and runs transversely into the root of the lung and divides into 

 three branches ; the left bronchus passes through the arch of the 

 aorta to the root of the left lung, and divides into two branches. 



The bronchi consist of cartilaginous rings, but as these tubes 

 advance into the substance of the lung, they diminish in size and 

 firmness, until their place is supplied by fibrous tissue, or transverse 

 circular fibres, which tissue also disappears, and at length nothing 

 remains hut the mucous membrane, which terminates in the air-cells, 

 upon which ramify the ultimate ramifications of the pulmonary 

 artery and the commencing radicles of the pulmonary veins. 



The ramifications of the pulmonary artery communicate with 

 those of the pulmonary veins, beneath the mucous membrane of the 

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